Process for the controlled deposition of silicon dihalide vapors onto selected surfaces



- 2,840,439" I DEPOSITION in'g'th'e lower halide vapors ---selected surface of a low fiow rate sothat-the exothermic *"decompiosi'tion fathe vaporscauses -acontinuous-crystal- PROCESS FOR THE CONTROLLED lizedlayerofthe-elementto be deposited thereon while OF SILICON DIHALIDE VAPOR ONTO SE- v- LECTED SURFACES 5 ulilglrlllilttilcotlfly liberating h gher halide vapors of the ele CharlesP. KempteraandClaildioAlvarez-Tostado, Stanford, Calif., assignors to Owens-Illinois Glass Company, .acorporation of Ohio N0 Drawing. Application January1'7 1 Serial No.- 559,539 6 Claims. or. 117-106) This invention relates toiamethod of udepositinguan 1 element ontoa selected surface and, moretparticularly, relates to ametho-dlfor the controlled deposition of. :an

- element onto a selected surface to form a crystallized :layer 'ofdesired thickness thereon, J0me of the known processesnfor acoatingl an .element onto a surface is: by electroplating:.whereinrgthe 'article Twhose 'SUIfiaCQ'iStQ be coated is placed in angelectrolytic :solution containing {the scoating element and thenilowzof current through the solution and-article.causesv azthin coatingoftheelement in solution to besdeposited onto t-he surface of the-article. 'Sincethe mass-of :anysubstance which is liberated at, :an electrode 'during "electrolysis is proportional to thequantity 10f electricity which passes through the electrolytic eell, there maybetaicertain control tover'the quantity; ofathe elementdeposited, 3 and, correspondingly, the thickness of the; coating ;on:the rsurface. =Electroplating, howeverghas.iseveral; obvious disadvantages. The primarydisadvantage is that azsurface which is non-zconductive to electricity.-:cannotzzbe v coated electrolytically. 1 Surfaces of; glass, resin 'ceraniics, certain plastic materialsaand thei'like, :WhiCh-gare. nonconductive, 'fall within this category. .rAnother ;disad vantage in electroplating :an article issthe rcost iinvolved inadfipositing a layer of;;an element'rof desiredzthickness 2011120121 metallic surface,isince itrtakes 96,5001coulombs4of electricity to liberate a gram-equivalent rweight-iof iany Tmetal from'a compound by electrolysis.

Another known ,process of icoatingzthe'surfaceof an article is by dipping. the articlezinto asolutionioribathof the coatingmaterial. Although articles having-.non-coniductingsurfaces may thus-becoated; there 'can, tof:'course, beno accurate control over thevthickness of the coating ,on thetsurface jof theg-article. The same holds true-for V spraying,rbr-ushing and other mthOdS:.Of coating surfaces 5 of. articles. l p Therefore, anobject of, this invention .isltotobviate the (above disadvantages Which are, ipresent-in existing coating uprocesses.

Another object of this;inv,ent

. ;,proved method. of :coating an articlewith-fa welbc 'lizedtlayernof an-telement. I I I "Still another object "of this invention is to provide a method of depositingra coating of anc eleme t of -de'sired thickness ontoa selected surface.

A further object of this invention is to provide for depositing a thin crystallized layer ofidesireddhick- 'ness of an element onto, a selected surface by exothermically decomposing an unstable lower halide ofgtthe -ele- =rnentontotheselected'surface'k v j Still another object of :this invention is to c'o'at' the vsurface-of anqarticle with a.continuouscrystallizedlayer lofan element, which layer tenaciously adheres thereto. I In attaining the objects of 'thisinvention chef-feature ;resides inlpassingllower halide vapors of the element :overthe selected surface and decomposing these vapors .aover the-surface so as"to-deposit-ajcrystalliied layer of the element thereon. Another feature resides in passrystalion 7 isl-toprov ide an im- 55 Still another feature resides informing the" lower halide vapors of the-elementat a'n elevated temperature'}wherein -th'e vapors are-stable and then-coolingth'ese'lower halide vapors-while simultaneously passing theinover andin contact'with a selected surface 'to'decomposethe 'vapors :into the free element and' higher 'halide vapors ofj the element whichuare sta'ble at the lowertemperaturfand cause the free element to'be deposite'd onto the surface. 5 'Yet-another feature reside's in passing "the; heated; stable lo'w'er halide vapors of the element 'overand in contact wvith a selected surface of known 'area at a predetermined fiowrate whichis measured by' the weight of the-higher halide *oi' the-element-whi'ch is reaeted'with the element -per-unit-of time to "form the lower halide vapors off the element w-hich are then klecom'posed over and -in contact withthe surface to forrn' acrystallized layer of theelementof desiredthickness'-thereon. t 0tbenobjectsjfeatures and advantages 'of -"this process -wi-ll become more apparent from' t-he' renewing-oesopti'o' of the-invention; a 1

Basicallygthe process includes thesteps-of decomposing normally unstable halideivapors of -an*elenient, such as lower halide vapors which are stable only a't anele- --of an article to' be coatedythus depositing a fine,,xt-hin, crystallized' layerofuhe element thereon -whelrthe lower halide va-p'ors are coole'd and i thus decomposed; ff

Using silicon as illustrative of aim-element which is to be coated onto a surfaceflt ,isbelieved that the following reaction occurs when silicon dichloride vapors which are stable tonly tat elevated temperatures are: cooled land ode omposez i 2 Sil (-vapors)' Si-i-SiCl; (vapors) +heat" .lRor every 72 molecules-of SiQl -.which are decornposed, ancatom wof. silicon is deposited onto the rselectedrsurface, landl-a. molecule ,of -.Si Cl vaporsis formed, 'Ihis'datter compound, ,which is stable at room temperature; mayr be V ;utilized.in-forrning additionali-lower silicon.halide-vapors by reacting it with a siliconbearing material, 'lOlrCVGH silicdn itself, 1 at elevated -temper-a-tures; wThe reaction 3 occurs asfollowsz 1 W a s r fHeat .11 0 SiCMvspors) +.-'si i2siontva ors) p llFromtthe outermostelectropic arrangements ofggrenps HI, IV and V elementguinmayglie-predicted;that losver halides of these elements can be produced at elevated temperatures as follows:

' I 1 p- I; X3H@2M.=

GroupIV 1MX +M= 2MX I vGro'npllv; '.3MX +2M;5-M'X ,nu m' gallium,;wi

a 7 a 18s 'thev-heated lo ershalidervapors of the relementapa'ss gover and .come, in direct contact .withthe;coolerirseteeted ls l'r fflcet at, a--controlled' low flow.-rate,ua :continuous crystallized layer of the element is deposite'drthereon duef'to tlie decomposition of the lower halide at the cooler temperature. By using silicon as an example of the element to dium,-- "arsenic, antimony,:- bismtith," and {be deposited: ontora surface i and .since :the .equilibriurn Patented 'JunzA t vated temperature; overan'din contactiw-ith the surface gen. j

0.1 to 0.2gram per hour.

lected surface. Thus, articles or materials having melting points inexcess of this, temperature may readily be used as i for; the deposited element. Those, materials .0: c s ha i a m s ri q t nsrq n that t 1 m m m. equi b i m eq ation 'l emper t e. sa m t, b so edbr th p c s r al h v nsmel ins pa stishfly; low t n the temp at re of t e rea s mi ture can. becoated if, the materials are in massiveform criifthcy re place o d; c n o s t a the s rfscet mpc am w the a er als. n chi m In insta ces. c u io of th depo t el me to. the selected surface occurs and maybe desirable. Tlhetthickness, of thelayerof the element deposited onto. a givenl surface is veryaccurately controlled by either setting the, ,residence ,time, for, .by controlling the weight of lower :halide per unit time passing over a seat ,800 C., this temperature would be the rnini-" m m pera thesa s i s m ture k n t a lectedgsurfaceuof specific area An inert carrier gas, such a'sheliuin, nitrogenand the likesis used to vary the ,per-

vapors may bedetermined by reactingaknownweight of 1 thehighcn halide of the element with the element per unit of time, thus forming the lower halide vaporsof the particular element. If the flow rateof lower halidevapors is too slow or too fast, the quantity of higher halide of theelement which is'toreactwiththe elementiscorthe lower halidevapors is toohiglLa-powdery layer'of .the element is formed on the selected surface rather than the co tinuous crystallized layer obtained bya Jmflwm i The followingexamples coating asurface depositing thereon alayer of an element formed by the decomof unstable lower halide vapors of the element are merely illustrative of the process and are;not to be considered as, limiting the scope of the invention.

QiExamplel V A mixture. of lower silicon dichloride vapors and helium was formed by passing the helium at a controlled low flow rate over silicon tetrachloride atOf C. and then passing cent or weight jot-lower halide fora given flow rate of the gaseous inixture.- The flow rate of the lower halide the mixture overimpuresiliconat 1220. C. The free silicon inthe latter reacted with the silicontetrachloride to form silicon dichloride vapors. A tantalum surface which was to be coated with silicon was placed adjacent this -high,tempejrature reaction zone and the mixture of silicon dichloride and, heliumfwas passedF- over and in contact with the cooler tantalurnsurface at a'low flow ratet Decomposition of the unstable silicon dichloride resulted in a continuous adherent crystallized layer of puresiliconpeingdeposited upon the tantalum surface, The resulting silicon tetrachloride whichwas formed by V the'decomposition of the silicon L removed and collectedin a condenser. 1

dichloride vapors" was 1 Example? it was at 125 to form a' gaseous mixturecontainingjlfi; by volumeof ume was passed over commercial silicon at 1190 C. to

form silicon difluoride vapors which were passed through and in contact with the inner walls of a tantalum cylinder placed adjacent the high temperature zone. Decomposition of the silicon difluoride vapors resulted in a thin layer of silicon being deposited on the inner walls of the cylindcr. The resulting SiF vapors were recovered.

' I ,"Example 4 element which is unlike the'surface (for example, a glass surface being coated with a'metal), it is also possible to coat a surface with an element similar to'that of the surface (for example, a silicon surface can be coated with a layer of silicon). Sillimanite surfaces have been successfully coated with extremely adherent coats of silicon up to approximately 3 mm. in thickness. An important consideration in the selection of the surface material is its reactivity with the depositing element at the decomrespondingly increased :or decreased. If the flow rate of position temperature. For example, silicon reacts only slightly with tantalum at high temperatures and, therefore, a layer of silicon may be deposited on a tantalum surface. However, if reactivity between the element and the surface were not desirable, then it is only necessary to select the materials with care so that such a reaction does not occur. In some cases, a reaction layer of demarcation might be desirable on the surface. The rate of deposition for the formation of a continuous layer of an element on a selected surface is quite low and, in fact, several hours are required to build up a thin layer'of silicon. This rate increases, however, as

the temperature of the equilibrium mixture increases and the temperature of the collecting surface decreases.

A silicon surface can be readily and rapidly coated with silicon because of its high melting point. At a temperature. of 1400. C. for the equilibrium mixture, a high deposition rate of silicon upon silicon is produced even at the low flow rate required for this particular process.

Among the uses for this process is the coating of sensitive electrical components such as n-p-n junction transistors. Also controlled coating of glass with metals and non-metals or the controlled coating of crystals is possible by this process.

Glass and ceramics may be coated for absolute inertness to hydrofluoric acid. Surfaces may also be coated witha thin layer of silicon in order to make silicon tetrachloride and this mixture was passed over and :in contact with commercial siliconat 1200'? C. to form silicondichloride vapors. A tatalum cylinder was placed immediately adjacentthis high'temperature zone and the exothermic decomposifion of the silicon dichloride vapors caused. ithefdeposition of a 'well-crystallized diamondf'cubic silicon layer on the inner surface of thetatalum cylinder. The flow rate was in excess :or one gram of liquid silicon tetrachloride, per hour,.and a layer of a silicon one millimeter in thickness was deposited in 72 hours at a rate of Example 3 i Agaseous mixture of 18% SiF; and 32% by. V9

. silicon for use as solar batteries.

them inert to acids such as HCl, H 80 and the like. The above processes are particularly'adapted for coating the. inner surfaces of cylinders. Metals and other materials may also be coated with a photosensitive layer of While. only several. specific embodiments of this invention have been described and only several specific elements have been listed, these examples are merely illustrative of the invention and are not to beconsidered as a limitation thereon. It is intended inthe appended claims to include all such processes which fall within the scope of this invention. l

.j i Having described fully. the invention, what is claimed 1 The process of coating a thin well-crystallized continuous layer of silicon onto a surface capable of withstandingwithout physical change thereto a temperature ,which at least as high as the minimum equilibrium temperature of silicon dihalide vapors comprising the 3 step of passing heated silicon dihalide vapors over and in contact with said surface at a predetermined flow rate, the temperature of said surface being relatively cooler than the temperature of said vapors and causing'exothermic decomposition of said silicon dihalide vapors into free silicon and silicon tetrahalide vapors, said free silicon being deposited onto said surface.

2. The process of coating :1 thin well-crystallized continuous layer of silicon onto a surface capable of withstanding without physical change thereto a temperature which-is at least as high as the'equilibrium temperature of silicon dichloride vapors comprising the step of passing heated silicon dichloride vapors over and in contact with said surface at a predetermined flow rate, the temperature of said surface being relatively cooler than the temperature of said vapors so as to exothermicallydecompose the silicon dichloride vapors into free silicon and silicon tetrachloride vapors, the latter vapors being removed from said surface while. the free silicon is deposited onto said surface.

3. The process of adhering a thin well-crystallized layer of silicon onto a surface capable of withstanding without physical change thereto a temperature which is at least as high as the minimum equilibrium temperature of silicon dihalide vapors comprising the steps of reacting j silicon tetrahalide with silicon at an elevated temperature j to form silicon dihalide vapors, and passing said vapors over and in contact with said surface at a predetermined flow rate while simultaneously cooling said vapors, the a resulting exothermic decomposition of said silicon dihalide vapors forming free silicon and silicon tetrahalide vapors, said free silicon adhering to said surface.

4. The process of adhering a thin well-crystallized layer of silicon onto a surface capable of withstanding without physical change thereto a temperature whichis at least as high as the equilibrium temperature of silicon dichloride vapors comprising reacting silicon tetrachloride with silicon at an elevated temperature to form silicon dichloride vapors, and passing said vapors over and in contact with the surface at a predetermined flow rate, said surface being relatively cooler than said vapors whereby said silicon dichloridevapors exothermically decompose into free silicon and silicon tetrachloride vapors, said free silicon being deposited onto said surface while said latter vapors are removed therefrom.

5. The process of coating a surface of known area with a crystallized layer of silicon of predetermined thickness, said surface being capable of withstanding without physical change thereto a temperature which is at least as high as the minimum equilibrium temperatureof silicon dihalide vapors, comprising the steps of reacting a known weight of silicon tetrahalide with silicon at an elevated temperature to form silicon dihalide vapors which are stable only at said elevated temperature, passing said silicon dihalide vapors over and in contact with said surface lected surface having a melting point just below the mini- 'J mum equilibrium temperature of silicon dihalide vapors comprising heating said surface to just below itsmelting point temperature and passing silicon dihalide vapors over and in contact withsaid surface at a predetermined flow rate While simultaneously cooling and exothermically decomposing said vapors to form the freeelement and silicon tetrahalide vapors, said element being deposited onto and fusing with said heated surface.

References Cited in the file of this patent UNITED STATES PATENTS I 2,438,892 Becker Apr. 6, 1948 2,470,306 Gross e May 17, 1949 2,746,888 Ross May 22, 1956 

1. THE PROCESS OF COATING A THIN WELL-CRYSTALLIZED CONTINUOUS LAYER OF SILICON ONTO A SURFACE CAPABLE OF WITHSTANDING WITHOUT PHYSICAL CHANGE THERETO A TEMPERATURE WHICH IS AT LEAST AS HIGH AS THE MINIMUM EQUILIBRIUM TEMPERATURE OF SILICON DIHALIDE VAPORS COMPRISING THE STEP OF PASSING HEATED SILICON DIHALIDE VAPORS OVER AND IN CONTACT WITH SAID SURFACE AT A PREDETERMINED FLOW RATE, THE TEMPERATURE OF SAID SURFACE BEING RELATIVELY COOLER THAN THE TEMPERATURE OF SAID VAPORS AND CAUSING EXOTHERMIC DECOMPOSITION OF SAID SILICON DIHALIDE VAPORS INTO FREE SILICON AND SILICON TETRAHALIDE VAPORS, SAID FREE SILICON BEING DEPOSITED ONTO SAID SURFACE. 